A half-bridge includes two switching elements each having a drive input and a load path, and having their load paths connected in series with each other between terminals for a positive and a negative supply potential during operation. An output to which a load may be connected to is arranged between the load paths. Using the half-bridge an alternating voltage at the out-put of the half-bridge can be generated from a voltage between the positive and the negative supply potential by alternatingly switching on and switching off the two switches. The two switching elements are driven by two drive signals each of which being provided to the drive input of one of the switching elements. These drive signals may assume an on-level for switching on the corresponding switching element and an off-level for switching off the corresponding switching elements.
There are circuit applications in which the half-bridge is driven in the ZVS mode (ZVS=Zero Voltage Switching). Such circuit applications are, for example, lamp ballasts, in which the load connected to the half-bridge includes a series resonant circuit and a fluorescent lamp (gas discharge lamp). Generally, the load in such ZVS circuits is capable of recharging the output of the half-bridge after switching off the first one of the switching elements and before switching on the second one of the switching elements, namely from the supply potential, to which the first switching element is connected to, to the supply potential, to which the second switching element is connected to. If the second switching element is switched on after recharging the output, than when switching on the second switching element there is no voltage drop across its load path. Therefore, the second switching element can be switched not energized, or at a load path voltage of zero (zero voltage), resulting in a reduction of switching losses. Correspondingly the second switching element can be switched not energized after switching off the second switching element.
The time period between switching off one switch and switching on the other switch is referred to as dead time. This dead time should be adapted to the recharging time of the output: If the dead time is less than the recharging time, then zero-voltage switching is not performed, resulting in increased switching losses; if the dead time is longer than the recharging time, then a current direction of the load current could revert before the end of the dead time, resulting again in an increase of the load path current at the individual switching element. In this case the switching operation would not be not energized, also.
However, the recharging time is dependent on different parameters, such as the load or the applied supply voltage, so that a fixed adjustment of the dead time for driving the half-bridge switching elements is not possible. There is therefore a need to adaptively adjust the dead time during operation of the half-bridge.
For these and other reasons there is a need for the present invention.